You could check the door/window seals or the rubber grommets in the floor ( if it has them ) try lifting or removing the carpet sit in the car while someone plays a hosepipe over the doors/ windows to see if you can spot it it could be a leakingheater matrix although you'dsee that in your coolant levels

Hi I have the same problem.... I have found the water is getting into the car from the boot so check if your boot mat is wet.
On mine water seams to be coming in by the light running along the top of the floorpan under the rear seat and onto the floor in the back.. I have been told its a common fault due to the rubber seals on the light drying out or going hard and not creating the correct seal.. silicone sealer is what im going to use on mine over the weekend if it doesn't work ill repost on Monday. ... Hope this helps (had similar problem with escort, orion & scorpio over the years all self solved the same way) if you do it.... dont forget to dry the car out properly first and only use silicone on the back of the lights out of sight when the unit is replaced